1. Field of the Invention
The present invention relates to an image reading apparatus for optically reading an image and a computer readable storage medium which stores program codes for realizing the function of the image reading apparatus, in particular, to the apparatus and storage medium suitable for reading of an image of an X-ray film of medical use.
2. Related Background Art
Various types of image reading apparatuses have been developed recently. Also in the medical field, apparatuses have been developed which detect an image, particularly an image of an X-ray film of medical use and digitalize it for use such as electronic filing, remote diagnosis and automatic diagnosis.
With such an image reading apparatus, light from a light source such as a halogen lamp and a fluorescent lamp is applied to an X-ray film, and light transmitted through the X-ray film is received by a solid state image pickup device and converted into an electric signal which is then converted into a digital signal to be output to an image processing circuit for shading correction and the like. The image pickup device may be a CCD line sensor and the like.
A CCD line sensor which is used as the solid state image pickup device has a variation in pixel data, called a dark output distribution (fixed pattern noises) caused by dark current fluctuation or the like. As a two-dimensional image is scanned and read with a CCD line sensor, the dark output distribution fixed to respective pixels as different from random noises appears on the image as artifacts which linear in the scan direction. Since a medical X-ray film image reading apparatus outputs density values, such artifacts become conspicuous in a high density area. Therefore, if a CCD line sensor is used as the solid state image device, it is necessary to check fixed pattern noises at each pixel and correctly compensate for such noises.
In a know method of compensating for fixed pattern noises, an output of each pixel of a CCD line sensor when a light source is not turned on is stored in a memory or the like, and an offset value corresponding to the fixed pattern noises is subtracted from pixel data by a subtractor. In order to suppress the influence of random noises, an output of each pixel picked up without light may be collected a plurality of times to use an average of the plurality outputs as fixed pattern noise correction data.
With this method, however, if random noises are mixed with the fixed pattern noise correction data, an image is corrected by using the fixed pattern noise correction data containing random noise components, and these noises appear on the image as linear artifacts.
If fixed pattern noises are collected a plurality of times and these noises are averaged in order to suppress the influence of random noises, it takes a long time to collect data, memory means capable of storing data of a plurality of lines becomes necessary, and the circuit structure becomes complicated.